Using one of the Keck Observatory’s 10-meter adaptive-optics-equipped telescopes, Imke de Pater and graduate student Ned Molter (both at University of California, Berkeley) spotted a bright storm complex spanning at least 30° in both latitude and longitude centered near the planet’s equator. The storm brightened between observations taken on June 26th and July 2nd.

The storm was initially thought to be the same Northern Cloud Complex first seen by the Hubble Space telescope in 1994, after the Great Dark Spot imaged by the Voyager 2 spacecraft in its 1989 flyby had disappeared. But measurements of this storm’s location show it to be something completely different.

A large, high-pressure vortex system deep within Neptune’s atmosphere is thought to drive the white storm clouds. As methane gases rise up in the vortex, they cool below the condensation temperature, forming clouds in the same way that water vapor does on Earth.

The location of the vortex caught astronomers by surprise, though. “Historically, bright clouds have occasionally been seen on Neptune, but usually at latitudes closer to the poles, around 10 to 60 degrees north or south” says de Pater. “Never before has a cloud been seen at, nor close to the equator, or anything so bright.”

Neptune is the windiest planet in the solar system, with observed equatorial wind speeds of up to 1,000 miles per hour (450 km/s). Since wind speeds vary drastically with latitude, a storm crossing more than 30° of latitude should quickly break apart. Something, such as an underlying vortex, must be holding it together. But a long-lasting vortex right at the equator would be hard to reconcile with our current understanding of the planet’s atmosphere.

It’s possible, given the storm’s extent, that it’s not a vortex after all but rather a huge convective cloud, similar to the one spotted on Saturn in 2010.

“This shows that there are extremely drastic changes in the dynamics of Neptune’s atmosphere, and perhaps this is a seasonal weather event that may happen every few decades or so,” de Pater says.

Neptune orbits the Sun every 160 years, with each season lasting 40 years. We’ve only had a close look at the planet for less than 30 years.

Amateur Observations

The Neptune storm captured on June 10 by Darryl Pfitzner Milika.

Some intrepid amateurs also detected the storm in the weeks preceding the Keck observations using telescopes of 10 inches or more. This storm hasn’t been seen at visible wavelengths so far, but experienced planetary imagers using high-speed video cameras and near-infrared filters have a good chance of capturing the storm (appearing as a bright off-center spot) on the diminutive disk of the planet.

Read Keck Observatory’s press release here.

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